If Alice was born in January and Bob was born in December, she will be 11 months older than him when they start going to school (and their classmates will be in average 5.5 months younger than her and 5.5 months older than him), which I hear can make a difference.

I think this way of sorting classes by calendar year of birth might also be six months shifted in different hemispheres (or perhaps vary with country in more capricious ways). IIRC, in Argentina my classes had people born from one July to the following June, not from one January to the following December.